FIELD OF THE INVENTION
The invention relates to a wood glue powder, processes for its preparation, and its use.
EP-A 311873 (U.S. Pat. No. 5,087,649) discloses pulverulent mixtures of starch and cellulose ether, comprising water-dispersible or water-soluble vinyl acetate polymers or vinyl alcohol polymers, for use as wallpaper pastes. The addition of the redispersible polyvinyl acetate powders is made in that case to reinforce the bonding strength. The process claimed indeed makes it possible to obtain adhesives whose bonding strength is sufficient for gluing wood; however, the requirement criteria to which a water-resistant wood adhesive For the interior sector is nowadays subject, especially for use as a furniture adhesive, are increasing, these criteria being high thermal stabilities and rapid setting in the compression process, in order to make it possible to carry out rapid machine bonding. Glue powders based on vinyl acetate homopolymers and/or water-soluble, partially hydrolyzed polyvinyl alcohols are inadequate in fulfilling these criteria.
JP-A 61-168681 (Derwent Abstract AN 86-241053) relates to pulverulent wood adhesives which are obtainable by spray drying polyvinyl acetate homopolymer dispersions which have been polymerized in the presence of partially hydrolyzed polyvinyl alcohol as protective colloid. To improve the bond strength, melamine-formaldehyde condensate is added to the powder. Powder mixtures of this kind do indeed form water-resistant bonds, but the addition of moisture-sensitive crosslinking agent renders their stability on storage and their blocking stability unsatisfactory. Furthermore, when polyvinyl acetate homopolymers are used it is necessary to add film-forming auxiliaries in order to attain the low minimum film-forming temperature of from 0 to 5.degree. C. Which is required for the application.
DE-B 2214410 (U.S. Pat. No. 3,883,489) and DE-B 2614261 (GB-A 1569637) relate to processes for producing redispersible vinyl acetate/ethylene dispersion powders which are prepared by emulsion polymerization in the presence of partially hydrolyzed polyvinyl alcohol as a protective colloid followed by spray drying of the dispersion. The powders are described as being suitable binders for adhesives and coating compositions. Because of the content of ethylene in the polymer it is, admittedly, possible to dispense with the addition of film-forming auxiliaries; nevertheless, to improve their blocking stability the powders contain up to 30% by weight of antiblocking agent, leading to a drastic decrease in the bond strength. Furthermore, the thermal stability of ethylene-containing vinyl acetate copolymers, whose ethylene content is relatively high in order to reduce the minimum film-forming temperature, is very low indeed.
DE-A 1794123 (ZA-A 6805680) describes an adhesive dispersion for bonding ceramic materials which comprises vinyl acetate homopolymer and high molecular mass, fully hydrolyzed polyvinyl alcohol. In order to improve the adhesion properties it is recommended to add plasticizers, for example dibutyl phthalate. A disadvantage in this case is that when volatile plasticizers are used the plasticizer evaporates after processing. In the case of wood glues, which are employed primarily in the domestic sector, the emission of volatile compounds must be avoided. Furthermore, although these mixtures can be spray dried, the powders obtainable therewith are not redispersible owing to the poor water-solubility of the high molecular mass, fully hydrolyzed polyvinyl alcohols. This rules out their use as wood glue powders for the production of aqueous wood adhesives.
Against this background the object was to develop pulverulent adhesives for bonding wood, whose use in aqueous wood adhesives leads to water-resistant bonds of high thermal stability and with rapid setting properties.
Owing to the tightening of environmental regulations, especially those arising from what is known in Germany as the Packaging Ordinance, the intention is to provide a pulverulent wood adhesive which can be transported and packaged in easily disposable paper bags, rather than the bucket packs of liquid dispersion adhesives used to date, and need only be stirred together with water prior to application. For transportation of palletized, bagged product, however, the requirements placed on the blocking stability of wood glue powders are stringent, especially for export to countries with a hot climate. A further object, therefore, was to provide pulverulent adhesives which couple high blocking stability with a low or zero content of antiblocking agent and which exhibit outstanding redispersibility in water.
One reason why wood glue powders have not to date found application in the production of water-resistant bonds is that in order to attain high bond strengths at low temperatures it has been necessary to add volatile film-forming auxiliaries to the polyvinyl acetate-based dispersion adhesives produced from powders. An additional object, therefore, was to provide wood glue powders which when used in aqueous adhesive dispersions have the required low minimum film-forming temperature even without the addition of volatile film-forming auxiliaries.